Individual carbon nanotubes (CNTs) are at least one order of magnitude stronger than any other known material. CNTs with perfect atomic structures have a theoretical strength of about 300 GPa (the term strength refers to tensile strength). In practice, carbon nanotubes do not have perfect structures. However, CNTs that have been prepared have a measured strength of up to about 150 GPa, and the strength may improve upon annealing. For comparison, Kevlar fibers currently used in bullet-proof vests have a strength of about 3 GPa, and carbon fibers used for making space shuttles and other aerospace structures have strengths of about 2-5 GPa.